The present invention relates generally to a mask pattern and a method for setting an optimal illumination condition to the mask pattern, and more particularly to a mask pattern and a method for setting an illumination condition suitable of a method of exposing a mask that arranges a desired pattern and an auxiliary or dummy pattern (these terms are used interchangeably in this application) smaller than the desired pattern, by illuminating the mask (reticle) using plural kinds of light so as to resolve the desired pattern and restrain the auxiliary pattern from resolving on an object to be exposed via a projection optical system.
The projection exposure apparatus is used in manufacturing devices, such as ICs, LSIs and liquid crystal panels, using the photolithography. For improved resolution, the projection exposure apparatus has generally shortened its exposure wavelength and enlarged a numerical aperture (“NA”) in the projection optical system.
In general, the resolving power improves with a shortened wavelength of exposure light and increased NA, while the projection exposure apparatus, from its nature, resolves some patterns relatively easily but cannot resolve other patterns easily. In general, it may be said that a line pattern more easily resolves than a contact-hole pattern, and thus a width in the contact-hole pattern is larger than that of the line pattern in a semiconductor chip. It is therefore an issue in fine processing in the photolithography to make a fine contact-hole pattern. More specifically, exposure cannot easily obtain the resolving power, depth of focus (“DOF”), and exposure dose tolerance for contact holes. In addition, the contact holes have a much larger mask error enhancement factor (“MEF”) that is a degree to emphasize a mask error, than the line pattern, which hinders fine processing.
Accordingly, Japanese Patent No. 3,119,217 (corresponding to U.S. Pat. No. 6,150,059) as one solution for this problem discloses a method to improve resolving power and DOF to some extent by forming, among openings, auxiliary openings so dimensioned that they are not resolved by exposure. Japanese Laid-Open Patent Application No. 2002-122976 (corresponding to U.S. Patent Application Publication No. 2002-045136) provides more concrete embodiments to this method.
These methods are effective to resolve contact holes of about 200 nm as in their embodiments but insufficient to resolve currently demanded 120 nm contact holes. More specifically, when the resolving power to 200-nm holes is converted into resolving power k1 for exposure using a general exposure apparatus at the time of filing of these applications which uses, for example, a KrF laser light source and NA=0.6 to 0.5, k1 becomes between 0.48 and 0.40. This conversion is performed with k1=(hole diameter of a desired contact hole)/(λ/NA) as a variation of Rayleigh's equation: (hole diameter of a desired contact hole)=k1(λ/NA). This converted k1 value corresponds to the resolving power of 163 to 136 nm for exposure using a currently available general exposure apparatus which uses, for example, a KrF laser light source and NA=0.73. It is therefore understood that the methods disclosed in these applications may resolve holes with a size of 163 to 136 nm, but cannot resolve holes with a size of 120 nm or smaller. Incidentally, 163 nm and 136 nm may be obtained as follows: (hole diameter of a desired contact hole)=k1(λ/NA)=0.48×248/0.73=163 (nm) for k1=0.48, while (hole diameter of a desired contact hole)=k1(λ/NA)=0.40×248/0.73=136 (nm) for k1=0.40.